The present invention is generally related to optical coupling methods and systems for optical fibers. The present invention is also related to techniques and devices thereof for coupling a vertical cavity surface emitting laser (VCSEL) to an optical fiber. The present invention is also related to single-mode and multimode VCSEL devices. The present invention is further related to optical scattering devices and optical concentrators.
Optical fiber technology is used in a variety of applications such as telecommunication, computer, and medical applications. An important aspect of optical fiber technology is the coupling of an optical fiber to an optoelectronic device for transmitting information conducted by the optical fiber.
Optical fibers are typically arranged in a bundle of individual fibers and protected by a sheath. Such a bundle of optical fibers is often referred to as an optical cable. The light receiving and emitting ends of the optical fibers are housed in fiber ferrules. The fiber ferrule at the light-receiving end of the fiber can be coupled generally to a light-emitting device or source via an optical interface unit. Likewise, the fiber ferrule at the light-emitting end of the individual fibers is generally coupled to a light-detecting device via an optical interface unit.
A drawback of these systems is the large number of optical interface or coupling points through which the optical signals pass. In currently available systems, optical signals can pass through at least four interfaces, i.e., interfaces between the optical interface units and the light emission and detection devices and interfaces between the optical interface units and the fiber ferrules. Each time the optical signal passes through an optical interface, a portion of the signal is typically lost, thereby degrading the quality of the optical signals transmitted in optical fibers.
In addition, for convenience and space efficiency, it is generally preferred to arrange optical fibers parallel to a circuit board. Therefore, when a vertical light emitting or detection device such as, for example, a vertical cavity surface emitting Laser (VCSEL) or photo detector is coupled to optical fibers, the chip that includes the vertical light emitting or detection device is generally mounted perpendicular to the circuit board. The perpendicular orientation of the vertical light emitting or detection device can be accomplished by several means, e.g., flexible circuits using tape automated bonding (TAB), electrically patterned submounts connected to the circuit board, etc. However, flexible circuits and submounts are additional components that are often expensive. Furthermore, flexible circuits and submounts increase the electronic paths of the light emitting or detection devices and, therefore, can degrade their performance.
Communications systems employing optical fibers are thus well known in the art. These systems typically transmit data by using a light source, such as a laser, to emit pulses of light onto a waveguide. The waveguide, often implemented as a glass fiber, transmits the light pulses to an optical receiver that senses the pulses of light and provides a corresponding output signal (typically an electrical signal) to a receiving system.
Optical communications systems may span large geographic regions, or they may be implemented within single electronic components. Vertical cavity surface emitting lasers (VCSELs) have been recognized as being useful in small-scale communications systems. Indeed, it has been suggested that optical systems utilizing VCSELs may eventually replace many systems that currently rely upon copper wires to transmit electrical data signals. The advantages of optical communications systems over electrical systems commonly include high bandwidth and low signal loss, which often results as optical data signals travel through the fiber. Moreover, several optical fibers may be bundled together in a xe2x80x9cfiber arrayxe2x80x9d to form a communications channel that is capable of transmitting multiple signals simultaneously.
An important element of any optical communications system is a method of coupling light emanating from a light source into the waveguide. Typically, a laser light source can be coupled into an optical fiber in a xe2x80x9cheader blockxe2x80x9d arrangement. The most commonly used form of header uses the well-known xe2x80x9cbutt-couplingxe2x80x9d method. xe2x80x9cButt-couplingxe2x80x9d involves positioning the laser so that light is directly emitted into an end of the optical fiber. Typically, a substrate made of silicon, plastic, ceramic or another material supports the laser and at least a portion of the optical fiber. The xe2x80x9cbutt-couplingxe2x80x9d method is particularly suited for use with edge emitter lasers that emit photons in an elliptical pattern, with the vertical axis of the pattern being longer than the horizontal axis.
A common practice is to form a groove into the substrate to support the optical fiber. Although the groove often prevents lateral movement of the fiber, it also typically increases the difficulty in aligning the fiber with the light source since the elliptical pattern of light emanating from the edge emitter is substantially narrower in the lateral direction. The grooves must, therefore, be precisely placed or else significant amounts of light can be lost, thus degrading the transmitted optical signals.
A VCSEL, either single mode or multimode, may be coupled to optical fibers. The combination of VCSEL and fiber technology is important in fields such as telecommunications. Single mode optical fibers have some advantages over multimode fibers because the core diameter of a single mode optical fiber is much smaller than the core diameter of a multimode optical fiber. The inventors have found that single mode fibers can be useful when combined with laser, and in particular VCSELs, for high-bandwidth telecommunications applications. Single mode fibers are available that are less than 10 microns in diameter. A well-known single mode fiber is what is referred to in the art as the SMF-28, manufactured by Corning Corporation. Furthermore, fiber is generally available that is operable as single mode fiber at 850 nm and is generally about 2-3 microns in diameter. Utilizing a multimode VCSEL in an SMF coupling scheme, however, can result in a system that suffers from poor modulation characteristics and low optical power.
The coupling efficiency of, for example, a multimode VCSEL has been known to change as a function of current and temperature due to the multi-spatial mode nature of the multimode VCSEL. Mode selectivity of the optical intensity of either single mode or multimode VCSELs can lead to problems in the time domain, such as changes in the optical pulse shape and an increase in the noise on the optical signal. Additionally, the associated coupling repeatability to, for example, single mode fiber can be poor because of the structure in the spatial modes.
In summary, there is a general inability in the present field to achieve coupling of single mode or multimode VCSELs (without regard to mode selectivity) into a small diameter fiber with improved alignment tolerances. The present inventors have thus concluded, based on the foregoing, that a need exists for an improved non-mode selective coupling of VCSELs into small diameter fibers, whereby alignment tolerances are improved. The inventors have found that a diffusing surface or other focusing/scattering mechanism can be utilized to mix the optical mode structure of a coupling system. This solution, however, can also result in overfill of an optical fiber, leading to a reduction in the optical signal and promoting modulation problems. The inventors now seek to present solutions to overcome these and other problems in the art.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, one aspect of the present invention to provide an improved optical coupling method and system for coupling lasers to optical fibers.
It is another aspect of the present invention to improve the coupling tolerance of VCSELs coupled to optical fibers.
It is yet another aspect of the present invention to improve the coupling repeatability.
It is still another aspect of the present invention to scramble optical modes associated with multimode laser devices, such as a multimode VCSEL, utilizing an optical scattering surface to eliminate mode selection noise penalties.
It is another aspect of the present invention to scramble optical modes associated with multimode laser devices, such as a multimode VCSEL, using an optical concentrator to eliminate mode selection noise penalties.
The above and other aspects can be achieved as is now described. A plurality of optical modes associated with a laser, for example a multimode VCSEL, are scrambled, and the VCSEL laser light from the multimode VCSEL is generally focused on a fiber facet associated with the optical fiber. Thereafter, the multimode VCSEL can be coupled to the optical fiber to thereby improve coupling repeatability while reducing mode selective coupling of the VCSEL to the optical fiber. The optical modes can be scrambled utilizing an optical scattering surface. A multimode VCSEL can be coupled to the optical fiber by butt-coupling said optical fiber to the multimode VCSEL or by utilizing at least one additional lens for imaging on a particular spot. Additionally, such a particular spot generally comprises a size larger than a core of said optical fiber to thereby reduce coupling variations thereof. The optical fiber can include a single mode optical fiber. The optical modes can also be scrambled, utilizing an optical concentrator configured as a linear, conical or parabolic concentrator (among other shapes), which may be adapted for use in mixing optical modes.